The present invention relates to the shielding of high voltage (HV) cable systems.
The present invention relates more in particular to the Electro Magnetic Field (EMF) shielding of underground AC three-phase HV cables in joint bays, manholes and joint chambers, as well as in other places where, for any reason, the cables are spaced from each other for a length, such as to go round an obstacle in the path of the cables.
For the sake of brevity, joint bays will be mostly referred to hereinbelow.
In the present description and claims the terms:                “High Voltage” or HV is used to indicate voltages higher than 35 kV, i.e. it is broadly used to include “Extra High Voltage” (ENV);        “Shielding Factor” or SF is used to indicate the ratio between the magnetic flux density at a given point in the absence of any shielding, and when the cable system is shielded, computed for example at the peak along the cable system axis or at the nearest house or critical location, or at the local peak in a joint bay.        
High voltage cable systems are used to distribute electric power from a power generating plant, and generally comprise one or more cables, notably three cables for three-phase systems. Cable systems may be aerial (overhead), terrestrial or submarine.
High voltage AC cable systems are known to emit an EMF, which is believed to be a health hazard, especially in densely populated areas. State administrations and institutions pose severe limitations on the EMF emissions allowed for any cable system. The EMF decreases rapidly with distance from the cable system, and is therefore a concern substantially only in terrestrial cable systems.
In terrestrial cable systems, spans of cable systems are generally underground in trenches. The three cables of a three-phase cable system can be laid in flat formation, i.e. with the longitudinal axes of the three cables lying substantially parallel in a same plane, or in trefoil formation, i.e. with non-coplanar longitudinal axes of the three cables, arranged in triangular formation, at short distance with each other, preferably with contacting cable sheaths, so that the cable system has a cross-sectional shape resembling that of a trefoil.
At the ends, the spans of the cables of the cable system are jointed with specifically designed joints. This is normally made at a joint bay. In order to accommodate the joints and the required inter-joint separation, the cables of the cable system are more widely spaced apart, normally in flat formation, but in principle also in triangular formation. It is also customary, in order to shorten the inter-axis distances between the cables, to longitudinally offset the joints of the cables in the joint bay. In the case of a three-phase cable system, the three joints are sometimes arranged in a delta configuration. Sometimes, moreover, the mutual positions of the three cables of a three-phase cable system are changed from one span (trench) to the adjacent one at the joint bay, in order to lower power losses. This can result in an even larger joint bay being required. Although this cable transposition will no more be referred to hereinbelow, those skilled in the art will understand that the invention is applicable irrespectively of its presence.
Similarly, the cables of the cable system may be more widely spaced apart for a portion thereof, for example in order to go round an obstacle or for other reasons.
As it is known, the EMF emitted by cable systems increases with the circulating current—that is typically of the order of magnitude of several hundreds to a few thousand of amperes in HV cable systems —, and also increases with increasing inter-axis distance of the cables. As a consequence, it is generally maximum at joint bays, and other locations where the cables are more spaced apart than in trenches.
When a very low threshold of EMF emissions is requested—at very critical locations, such as near schools, kindergartens and similar —, metallic plates, steel pipes, and ferromagnetic raceways are normally used at the trenches, and ferromagnetic cases are normally used at the joint bays.
At other locations, an EMF not exceeding e.g. 3 μT when measured at 1 m above ground level is generally acceptable, and passive loop shielding techniques have been devised. Considering the rapid decrease of EMF with distance, a 3 μT EMF at the critical location is generally obtained when the EMF along the longitudinal axis does not exceed 10 μT.
In the article by Paolo Maioli and Ernesto Zaccone, published at Jicable 07, ‘Passive loops technique for electromagnetic fields mitigation: applications and theoretical considerations’, several passive loop shielding techniques are reported.
The passive loop technique is a method for EMF shielding, with Shielding Factor that can reach high values. Such technique provides for installing passive—i.e. not powered—loops of cables into the trench or in a joint bay in order to mitigate the EMF. Low Voltage (LV) cables are normally installed, due to the very low tension induced into the cables. The passive loops can be arranged on the surface of the compacted backfill—or above the level of the AC cables in a joint chamber —, at the same level of the AC cables, and/or below the level of the AC cables. Several loops can also be arranged, spaced about the cross-sectional perimeter of the joint bay.
In one proposed solution for a 345 kV cable system with a current of 1368 A, a layer of four loops of passive cables is installed, placed 400 mm above the joints. The cables, with a section of 300 mm2, are placed at positions of ±1 m, ±0.9 m, ±0.8 m and ±0.7 m from the joint bay longitudinal axis. The inner loops are 1 m longer than the length of the power cable in the joint bay and shield the part of the cables where they progressively recover the flat configuration. The EMF is shielded below 20 μT, is about 17 μT at peak level, and is 3 μT at about 6 m from the longitudinal axis of the joint bay.
This solution is installed in a joint bay and will be described in more detail later with reference to FIG. 14.
In other proposed solutions, the passive loops are said to be laid regularly spaced on top of the backfill starting from the extremities of the trench and gradually adding further cables towards the center.
In the article by Paolo Maioli and Ernesto Zaccone, published at Sarajevo Colloquium on EMF, 3-4 Jul. 2009, ‘Thermal design of HV electric systems with EMF mitigation devices’, a photograph of a passive loop EMF shielding of a 132 kV joint bay is shown. Current in the power cables is 860 A.
The article also reports the use of copper plates as providing a higher SF than passive loops. A flat plate for EMF shielding of a 87/150 kV cable in flat formation in trench may be placed above and/or below the level of the HV cables; more efficient solutions are said to use “H shape” or “inverted U” and also two vertical panels close to the sides of the trench.
Another EMF shielding of the passive cable type provides for an equally spaced configuration of parallel cable lengths, connected together at corresponding ends by two opposite terminal boards and covering the whole length and width of the zone wherein the power cables are spaced. This solution is described in more detail later with reference to FIG. 15.
In the present description and claims, the terms “conductive”, “insulated”, “connected” and other terms that might also have a thermal or mechanical meaning are used in the electrical meaning, unless otherwise specified.